Alum tannage



tannin (Granted under Tifl e SS, Us. code 1952 states A nonexclusive, irrevocable, royalty-free license in nited States Patent ()ffice Patented-lune 28, 1960 basicity, which may require the addition of a masking? agent such as sodium acetate, sodium citrate or sodium tartrate in the proportion of one mole of masking agent ,;to,,one mole of. aluminum sulfate, have also been used. The pH of thesolution may be varied from 4.2 to 6.5 and the time of treatment from 4 hours to 48 hours, depending upon thetypecf skins used and the kind or. leather desired. t-The amount-of aluminum in tlie finished leather, de'terrr-iined as A1 0 and usually 25 to;6%, .may be-controlledby varying the tanning conditions. The operation is conducted in a V drum, paddle,or byother suitable'methods. I i

The second part of fl-ieinventiVe process comprises treating the skins, after fa short wash in water, with a the invention herein described, throughout the world fior all purposesof the United States Government Withpower to grant sublicenses for such purposes, is hereby granted to the Government of the United States of America.

I This application is a continuation-in-part of our co pending application entitled Alum Tann'age, filed Qctober 30, 1956, Serial No. 619,363, now abandonedj [This invention relates to a newmethod of tanning skin .Ihe. tannage ofleather by the use of, aluminumisalts solution of the homopolymer of acrylic acid, or sodium polyacryiate or 'ammoniunr polyacrylate, containing approximately 2% polymerjbased on the wet drained Weight of the skins, for 8 to 24 hours. Commercially available polymer preparations, representing low molecular Weight polymer and high molecular Weight polymer, were employed in operating the process.

At thej-Iti'me the 'solutionot polymer is being applied thelpH :shouldpreferably'be in thelrange of about 4.5

is. an old, Well-known process. However, the *tannage' is cbmmonly known to be unstable, yielding leathers tof PQOr-heat and Waterresistance; It is quite reversible;

the loose'ly combined alum is poorlyfizgedinthe leather,

' ydrolyses readily, and is easily removed by washingwith 1 i [Ah objectof this invention is tomak'e a more stable? irreversible alum tannage. I Another object; of the present invention is to produce, an alumetanned leather which may be. fat-liquored-satis-.

factorily in aqueous fat-liquoring systems. similar to those employed in the leather-making trade.

A further object is to provide an alum-tanned leather which may be retanned successfully with vegetable In general, according to the invention, the skins are impregnated with a solution of a water-soluble aluminum salt until the skins are. tanned in a usual manner. The tanned skins are then subjected to the action of a homopolymer of acrylic acid, of sodium polyacrylate, or of ammonium polyacrylate to fix the aluminum salt in the. tanned, skins. The skins are thenfat-liquoredin anaqueous fat-liquoring treatment. similar to that employed in the leather making trade. A white leather is p o uc d- Although the initial appearance of skins which have been: tanned 'by. applying the same reagents, inreverse order, or by simultaneous application of the reagents,

indicates a tannage to give a somewhatcomparable white leather, quite unexpectedly only the the combination of. the; present. invention, that is, impregnating withv asolution of a water soluble aluminum salt followedby sub' jecting the tanned skins tothe action of a'hornopolymer of acrylic acid, of sodiumpolyacrylate, or of ammonium "polyacrylate, gives a leather whichupon'fat-liquoring in.

a typical. aqueous iatv-liquoring-system results in-a satisfact ory'product. The; finished leather resulting from thepresentfinvention is especially suitable fonupholstery, gloves,for other uses where a soft, flexible leatherisv he first par-t of the process, commonlyneferred to as an alum tannage, comprisesfiimpregnating the skins with a solution of a Water soluble aluminum salt, Commercially available basic aluminum acetate, such asaluminumasulfate, adjusted: to the" desired degree-of to 5.0.-Q. 7

"After tanning the standard procedure is to fat liquor the; tanned skins. f A dyeingstepirnay precede the fat:-

liquoring 'inicorrr'r'nercial practice. ifat.-liquoring 'jconif' of theseoils with a vegetable' oil such as coconut oil,

and rice bran oil. Several proprietary oils containing mixtures of the typical oils used in the trade were employed to satisfactorily fat-liquor skinsa tanned :by the process of thepresent invention. in all instances ,the area shrinkage'of the skin and loss of aluminum salt duringfat-liquoring .we-re', negligible and a strongsofh' flexible leather was obtained. In contrast, comparative skins tanned by applying theppolymer tothe skinprior to, tanning with the aluminum salt, or by applying the polymer. and the aluminum saltin the same solution, did not" fat-liquor.satisfactorily, resulting in hardleathers, oily on the surface, whic-h had shrunk excessively, some times asmuch as one third of the original area. Some typical illustrations or the application of the present invention are presented in the-following examples:

'(w./v.) NaCl for a p'eriod. of- 24 hours.

, Example I I A piece of pickled calfskin was drummed intermittently in a solution of a water soluble basic, aluminumacetate containing,4% A120 basedon the wet drained weight of skin 33 grams) and 10% (w./v.) NaCl. Float ratio was 3 to 4 times wet weight skin. After21 hours the pH, was 4.5.: :This was adjustedv'to pH 6.0-6.5 with borax and the drumming continued 3 hours. The skin was re moved, washed 5 minutes. inlukewarm water, drained,

and drummed intermittently in a solution of high meleeular. weight homopolymer'o'f acrylic. acid containing,,2'%.i polymer based on wet'drained weight of skin and 10% Float ratio 7 was about 7 times weight of wetskim. .The pH of the polymer' solu'tion was 3.10. at the st art and 4.50 after 24 hours? The 'leathetjwas Washed 1'0 minutes in -,l uk e'-f warm water; 'drainediandcut. in half. One half was fat liquor'ed-in an aqueous oil emulsion system, similar to that used in theittadei 'formin'eral tanned drum-ll E lining for jone and one-halfhours at F., removed, and tacike'dbht todry. The otherhalf was-diummed! intermittently in a 42 Bk; sulfited qu'ebrachosolution forf 20 hours, washed 2" minutes with water, 'f at-liquo redes; described, and tacked out ftodry. The alum-.polyacry icf; acid-tannage produced auniformly white, full; pliable,

well-tanned leather with an even, close grain, having a shrinkage temperature of 87 C. Retannage of this leather with sulfited quebracho produced a uniformly tanned, soft, pliable leather of good color and having a shrinkage temperature of 103 C.

' Example II 4 k 24 hours. Float ratio was 5 times wet weight. The pH of the sodium polyacrylate solution was adjusted with A piece of pickled calfskin was drummed intermittently in a solution of a water soluble basic aluminum acetate containing4% A1 based on the wet drain weight (51 grams)- and 10% (w./v.) NaCl. Float ratiowas 3 to 4 times wet weight. After 21 hours the pH was 4.5.

lactic acid so that at the end of the 24-hour treatment the pH was about 4.8. The leather was washed 10 minutes in lukewarm water, drained and cut in half. Onehalf was fat-liquored in a system conventionally employed with mineral tanned skins and tacked out to dry. The

other halfwas drummed intermittently in a 46.5 Bk. I

sulfited quebracho solution for 20v hours, rinsed 2 minutes with water, fiat-liquored and tacked out to dry. The

alum-sodium polyacrylate tannage produced a white, full,

well-tanned leather with a shrinkage temperatureof- 89 This was adjusted to pH 6.0-6.5 with borax and the drumming continued 3 hours. The skin was then removed, washed 5 to minutes in lukewarm water, drained and drummed intermittently in a solution of high molecular'weight sodium polyacrylate containing 2% polymer on wet drained weight skin and 10% (w./v.)

v C. Retannage of this leather with sulfited quebracho produced a leather that was run, soft, well tanned, evencolored and had a shrinkage temperature of 110 C.

n. h r e d s m l t e ran f ota 203 tent was from 3.2 to 5.5%; water soluble A1 0 0.3 to 1.6%; fixed A1 0 2.9 to 4.4%. For comparison a pickled calfskin control was tanned under the same conditions with a solution of awater soluble basic aluminum acetate at pH of 4.8 'but received no further treatment.

' Analysis of this control piece showed 4.6% total A1 0 3.0% water soluble A1 0 and 1.6% fixed A1 0 Comparison of the water soluble and fixed A1 0 values shows that the polymers ofacrylic acid and sodium polyacrylate bind the aluminum salts in the leather,'preventing them bracho solution for 2 0 hours, rinsed 2 minutes with water,

fat-liquored with a nonionic oil and tacked out to dry. The alum-sodium polyacrylate tannage produced a white leatherthat was quite. full, pliable and well tanned with a shrinkage temperature of 91 C." Retannage of this leather with sulfited quebraeho produced a plump, welltanned leather of uniform color with a shrinkage temperature of 116 C.

Example'lll f A piece of pickled calfskin was intermittently in a solution of a water soluble basicaluminum acetate containing 4% A1 0 based on the wet drained weight (51 grams) and 10% (w./v.) NaCl. 3 to 4 times wet weight. After 22 hours the pH was 4.6.

Float ratio was This was adjusted to pH 6.6 with borax and the drumming continued 4 hours. The skin was then removed, washed 5 minutes in lukewarm water, drained and drummed intermittently in a solution of low molecular weight polyacrylic acid containing 2% polymer on wet drained weight of skin and 10% (w./v.) NaCl for a. period of 24 hours. Float ratio was 5 times wet weight, The pH of the polyacrylic acid solution was 2.80 at the start and 4.79 after 24 hours. The leather was washed 10 minutes in lukewarm water, drained and cut in half. One-half was fat-liquored in an aqueous system containing a proprietary nonionic oil and tacked out to dry. Theother half was drummed intermittently in a 46.5 Bk. sulfited quebraeho solution for 20 hours, rinsed 2 minutes with water, fat-liquored with a nonionic oil and tacked out to dry. The alum-polyacrylic acid tannage produced a white, full, soft, pliable leather with a shrinkage temperature' of 87C. Retannage of this leather with sulfited quebracho produced a soft, plump, fully tanned leather from being hydrolyzedand washed out;

Vegetable tannin easily and quickly penetrated'the alum-polyacrylate tanned skin and produced a soft, pliable; fully-' tanned leather with a uniform color and with a shrinkage temperature well above 100 C. For the vegetable retanned leathers of Examples I and IV the range of total-A1 0 content was'fi-om 2.2 to 3.5%,

water soluble A1 0 0.2 to 0.3%,and fixed A1 0 2.0 to 3.2%. When a skin tanned only 'vvith a water soluble aluminum salt is retannedv with vegetable tannin, practhe weight of aluminum sulphate.

of even color and with a shrinkage temperature of 112 C.

I v Example IV V A piece of pickled calfskin was intermittently in a solution of a-water soluble basic acetate containing 4% A1 0 based on the wet drained weight (49 grams) and 10% (w./v.) NaCl. Floatratio, was

3 to 4times-wet weight. After 22 hours the pH was 4.6. This was'adjusted to pH 6.6 with borax and the drumming continued 4 hours. Theskin was then'removed, washed tically all of the aluminum salt is displaced by the vegetable tannin (0.1 to 0.2% A1 0 remains) and forms a sludge or precipitate on both surfaces of the skin, resulting in a very poor leather.

Example V A piece of Iranian sheepskin was drummed in a solution of aluminum sulphate containing 3% A1 0 based on the wet drained weight .of the skin (403 grams) and 10% (w./v. sodium chloride. times wet weight skin. The aluminum sulphate was masked with anhydrous sodium acetate; one mole Al (SO -18H O (79 grams) to one mole of anhydrous sodium acetate (9.7 grams). This solution was basicified with anhydrous sodium carbonate, based on A of A pH. of 4.10 was obtained after drumming the skin 1% hours. Suflicient anhydrous sodium carbonate was added in two feeds to bring the final pH to 4.8 after a 24-hour tannage. The skin was removed, washed for 5 minutes in water, drained and drummed in a solution of high molecular weight sodium polyac'rylate' containing 2% on the wet drained weight skin and 10% (w./v.) sodium chloride for a period of 24 hours. The float ratio was 3 times wet weight. The pH of the-sodium polyacrylate solution was adjusted with lactic acid so that the final pH at' the end of the tanning period was 4.9. The leather was washed 30 minutes in water and drained. The aluminum sulphate polyacrylate tannage produced a white leather that was full and well tanned with a shrinkage 1: "temperature of 80 C. and-with a highfixed A1 0 The aluminum{sulphate-polyacrylate tanned skin was fat-liquored with a commercial fat-liquor in the conventional aqueous. system. The entire system of fat-liquoring was conducted at F. The fat-liquor was well exhausted after 30 minutes to 1 hour drumming. The product from the f at-liquoring of the skin tanned by the The float ratio was 3' two step aluminum sulphate-polyacrylate process is a slightly oil-white colored leather. The oil penetrated well, making the product flexible and quite soft. Area shrinkage and loss of A1 during fat-liquor were negligible.

For comparative purposes the following examples are included to demonstrate the results obtained when the sequence of applying the aluminum salt and the polymer are altered.

Example VI A piece of sheepskin was tannned by a process similar to that described in Example IV with the exception that the basic aluminum acetate was equivalent to 3% A1 0 based on wet weight skin. Float ratios were 4 to 1.

A comparable piece of sheepskin was treated with solutions containing exactly the same concentration of reagents but the process was reversed, applying the solution of sodium-polyacrylate first.

Tanning process conditions, such as time skin was in each solution, pH, and temperature were the same in each instance.

The tanned skins were fat-liquored by conventional commercial fat-liquoring processes. Area shrinkage and loss of aluminum salt (determined as A1 0 was negligible for the skin from the process in which the tannage with aluminum salt was followed by application of the polymer solution. A satisfactory fat-liquored leather was obtained. Attempts to fat-liquor the leather from the process in which the alum tannage followed the application of the solution of polymer were unsuccessful, resulting in unsatisfactory leathers.

Example VII A pickled Iran sheepskin was cut along the backbone to give two comparable halves of skin.

One half-skin was tanned with a solution containing water soluble basic aluminum acetate equivalent to 3% A1 0 wet weight drained skin, 2% low molecular weight sodium polyacrylate (calculated on same weight skin) and 10% (w./v.) sodium chloride. This solution was adjusted to pH 4.8 with borax solution. The skin was I drummed according to standard procedure in this solutioncontaining both aluminum salt and polymer. Float ratio was about 4 to 1 and temperature 85 F. The pH dropped slowly to 4.5 over a period of about 5 hours. The pH was readjusted to 4.8 and drumming continued for a total of 24 hours. The tanned skin was washed, drained and fat-liquored by drumming for one and onehalf hours at 120 F. in a system similar to that used in the trade for mineral tanned skins.

The oil did not penetrate well into this skin, being on the surface, especially on the flesh side. The grain surface was coarse and rough and the leather lacked flexi bility. The shrinkage to about two-thirds the area considered normal for this process was excessive. This leather would be rejected in the trade. a The other half of the pickled Iran sheepskin was tanned according to the two-step process of the present invention. The skin was first drummed for 24 hours in a solution containing aluminum salt equivalent to 3% A1 0 based on wet weight drained skin and 10% (w./v.) sodium chloride adjusted to pH 4.8 and at F. The skin was then drummed in a solution containing 2% low molecular weight sodium polyacrylate based on wet weight skin and 10% (w./v.) sodium chloride, pH 4.8, at 85 F. for 24 hours. Float ratios were about 4 to l.

The tanned skin was washed, drained and fat-liquored substantially as described for the other half-skin. The oil penetrated well, making the leather flexible and quite soft. Shrinkage and loss of aluminum salt during fatliquoring were negligible.

In addition to those mentioned, the process of this invention may be applied to other types of skins, and to bated as well as pickled skins.

We claim:

1. A process for tanning skins comprising impregnating the skins with a solution of a water soluble aluminum salt until the skins are tanned, and subjecting the tanned skins to the action of a water soluble material selected from the group consisting of homopolymer of acrylic acid, sodium polyacrylate and ammonium polyacrylate, to fix the aluminum salt in the tanned skins, and subjecting the thus tanned skins to an aqueous fat-liquoring treatment.

2. A process according to claim 1, wherein the aluminum salt is basic aluminum acetate and the water soluble material is sodium polyacrylate.

References Cited in thefile of this patent UNITED STATES PATENTS Great Britain May 22, 

1. A PROCESS FOR TANNING SKINS COMPRISING IMPREGNATING THE SKINS WITH A SOLUTION OF A WATER SOLUBLE ALUMINUM SALT UNTIL THE SKINS ARE TANNED, AND SUBJECTING THE TANNED SKINS TO THE ACTION OF A WATER SOLUBLE MATERIAL SELECTED FROM THE GROUP CONSISTING OF HOMOPOLYMER OF ACRYLIC ACID, SODIUM POLYACRYLATE AND AMMONIUM POLYACRYLATE, TO FIX THE ALUMINUM SALT IN THE TANNED SKINS, AND SUBJECTING THE THUS TANNED SKINS TO AN AQUEOUS FAT-LIQUORING TREATMENT. 